Liquid droplet erosion (LDE) and cavitation erosion reduce the lifetime of industrial hydraulic machinery. Turbines, for example, include rotary engines having vanes that move through a liquid. The liquid erodes portions of the surfaces of the vanes and turbine housing that are contacted by the liquid. Industrial machines (such as turbines) are expensive, hence a need exists for making the machines more resistant to erosion of this type.
Cavitation erosion results from the growth and collapse of vapor cavities (bubbles) in a liquid due to dynamic pressure changes. The mechanism responsible for material loss as a result of LDE and cavitation erosion was poorly understood until recently. Over the last forty years, many attempts have been made to correlate material properties, or combinations of material properties, such as hardness, tensile strength, ductility, etc. with measured erosion. Unfortunately, these studies obtain good correlations only within narrow classes of materials having very similar structure. See, for instance, Heathcock et al., "Cavitation Erosion of Stainless Steels," Wear 81:311-327 (1982).
Hydraulic machines, such as hydroturbines and impellers, typically are made from plain-carbon structural steels and from stainless steel, such as CA6NM and Type 304L. Unfortunately, all of these steels also experience significant material loss when exposed to cavitation and LDE erosion. Hence, a need exists for developing materials that are superior to common structural materials for resisting erosion.
NiTi alloys also are known, as is the potential for such alloys to resist erosion. See, for instance, "The Influence of Microstructure on the Cavitation Erosion of Materials," Proc. 5th Intl Conf. on Strength of Metals and Alloys, (Oxford, UK:Pergamon Press, 1979). However, the surprisingly superior nature of certain NiTi alloys for resisting erosion was not known prior to the present invention. Moreover, a need exists for a method for producing erosion-resistant industrial machinery by plating structural materials, such as steel, with erosion resistant materials. However, known welding techniques often produce a welded material that is brittle. Thus, welding an erosion-resistant material to a structural material such as steel to form a composite structure may reduce the suitability of the composite for resisting erosion. Moreover, where only certain alloy compositions are erosion-resistant, fusion welding may vary the composition of the alloy, thereby reducing or eliminate the erosion-resistant capability of the composite structure. Thus, a need exists to (1) identify or develop new erosion-resistant materials, and (2) develop techniques for forming erosion-resistant industrial hydraulic machinery using the newly identified or developed erosion-resistant materials.